1. Field of the Invention
The present invention relates to a dishwashing machine that has a single-motor pump unit for both circulation and discharge of a washing liquid in the dishwashing machine.
2. State of the Prior Art
U.S. Pat. No. 4,799,855 discloses a dishwashing machine that is provided with a pump unit that has both a water circulating pump and a water drain pump. The impellers of the water circulating pump and the water drain pump are both keyed on a common shaft that can be reversibly driven by a drive motor. The water circulating pump is operated in one direction of rotation, and the water drain pump is operated in the other direction of rotation. The circulating pump is a so-called "action" centrifugal pump, and the impeller of the drain pump is a peripheral outflow impeller. The two pumps are separated by a partition, the partition having a calibrated annular opening defined around a hub which is common to the impellers of both pumps.
The above type of pump unit reduces remarkably the total energy consumption, the vibrations and the hydraulic noise occurring during operation of the dishwashing machine. The annular opening in the partition between the two pumps, providing a connection between the washing water circuit and the drain water circuit of the dishwashing machine, advantageously enables water collected in both of the water circuits to be discharged during every discharge phase.
However, during the washing operation the annular opening providing a direct connection between the drain water circuit and the washing water circuit causes an undesirable agitation of dirt particles deposited at a position corresponding with the suction side of the drain pump. It is preferable, as is known, that the dirt particles be allowed to stagnate during washing operations in order to prevent the dirt particles deposited at the suction side of the drain pump from polluting the washing water.
Furthermore, it is noted that during washing operations the impeller of the drain pump rotates in a direction opposite to its normal direction of operation. As a result, the pump chamber of the drain pump tends to fill with air. Consequently, at the beginning of each subsequent drainage operation the drain pump is in a substantially poor priming condition. This has an undesirable effect on the performance of the drain pump.